Cold Field Emitter

ABSTRACT

A stable cold field electron emitter is produced by forming a coating on an emitter base material. The coating protects the emitter from the adsorption of residual gases and from the impact of ions, so that the cold field emitter exhibits short term and long term stability at relatively high pressures and reasonable angular electron emission.

This application is a Divisional of U.S. application Ser. No.11/900,956, filed Sep. 14, 2007, which claims priority from U.S.Provisional application No. 60/897,369, filed on Jan. 24, 2007, whichare hereby incorporated by reference.

TECHNICAL FIELD OF THE INVENTION

The present invention relates to electron emitters, and in particular tocold field electron emitters.

BACKGROUND OF THE INVENTION

Electron emitters are used to generate electrons that are directed intobeams for electron microscopy and other applications. Electronmicroscopy includes scanning electron microscopy, transmission electronmicroscopy, and scanning transmission electron microscopy, as well asanalytical variations of these techniques. An ideal electron sourceproduces a beam of electrons that can be focused to an extremely smallspot with sufficient current to provide rapid, consistent datacollection. Such an electron source is typically characterized by lowenergy spread among the emitted electrons, high brightness, andlong-term stability.

To be freed from a solid surface, an electron must overcome an energybarrier. The height of this energy barrier is referred to as the “workfunction” of the material. Thermionic electron emitters are heated by afilament to provide the electrons with sufficient thermal energy toovercome the energy barrier and leave the surface. Schottky electronemitters use a combination of coating materials that lower the workfunction, heat to provide thermal energy, and an electric field to freethe electrons. Cold field electron emitters, on the other hand, use anelectric field to provide the conditions for electrons to tunnel throughthe energy barrier, rather than providing the electrons with thesufficient thermal energy to pass over the barrier.

Because cold field emitters provide high brightness with a small energyspread, they offer improved resolution for electron microscopy. Coldfield emitters are not commonly used in electron microscopy, however,because of both long term and short term emission instability. Shortterm stability refers to the ability to produce a constant emissiondistribution over a period in which an individual operation, such asforming an image, occurs. Long term stability or source lifetime refersto the ability to provide a relatively constant emission distributionfor performing many operations, typically over a period of hours ordays.

Although electron beam columns operate in a vacuum, the vacuum is notperfect, and some residual gas molecules are always present. Theresidual gases tend to adsorb onto the emitter tip, causing changes inthe emission characteristics. Moreover, electrons from the emittercollide with the gas molecules, creating positive ions that areaccelerated back towards the emitter by the electric field. The impactof these ions damage the emitter surface by sputtering material from thesurface, and the damaged surface changes the electron emissioncharacteristics. In Schottky emitters, which typically operate at about1,800 K, the emitter surface repairs itself over time, as atoms migrateover the surface. This “self-repair” does not occur in cold fieldelectron emitters, which operate at close to room temperature. Coldfield electron emitters are therefore heated or “flashed” periodicallyto allow surface atoms to migrate to repair damage and to removemolecules that are adsorbed onto the emitter surface. Heating the coldfield emitter, however, interrupts the operation of electron microscopeor other equipment in which the emitter is installed. Cold fieldemitters can be operated with an external feedback control loop thatdetects the beam current and maintains a constant beam current byincreasing the voltage applied to the emitter as the current decreasesover time.

Because cold field emitters rely on a very high electric field to emitelectrons from the surface, the emitters typically require a very sharppoint, that is, a tip with a very small radius, to achieve the requiredelectric field. The small emitting area of a cold field emitter causesmore short term variation in the electron beam because small variationsin the tip structure and random motion of adsorbed gases on the tip arenot averaged out over a large emitting area. Also, heating the emitterto clean the tip tends to blunt the tip, as atoms in the emitterrearrange themselves to reduce the surface energy. After heating the tipmany times, the tip radius increases to a point at which the radius istoo large for adequate field emission.

Schottky emitters typically operate at pressures in the 10⁻⁹ Torr(1.3×10⁻⁹ mbar) range. To improve the stability of cold field emitters,they are typically operated at a pressure of less than 10⁻¹⁰ Torr(1.3×10⁻¹⁰ mbar). The lower pressure reduces the amount of gas that isadsorbed onto the cold field emitter and reduces the damage from ionbombardment, thereby reducing the required frequency of flashing. Thelower pressure, however, is more difficult to achieve. Because of theinstability of cold field emitters, Schottky emitters, which operate athigher pressures and are more stable, have become the standard electronemitter for most high resolution microscopy systems and applications.

SUMMARY OF THE INVENTION

An object of the invention is to provide a cold field electron emitterwith improved stability.

This invention comprises a cold field electron emitter having a coatingthat provides improved stability. The invention also includes a methodof making a cold field electron emitter and a method of emittingelectrons. In a preferred embodiment, the coating is thought to reducethe adsorption of residual gases onto the emitter. A preferred coldfield emitter can operate at significantly greater pressure than priorart cold field emitters and provides improved stability.

The foregoing has outlined rather broadly the features and technicaladvantages of the present invention in order that the detaileddescription of the invention that follows may be better understood.Additional features and advantages of the invention will be describedhereinafter. It should be appreciated by those skilled in the art thatthe conception and specific embodiment disclosed may be readily utilizedas a basis for modifying or designing other structures for carrying outthe same purposes of the present invention. It should also be realizedby those skilled in the art that such equivalent constructions do notdepart from the spirit and scope of the invention as set forth in theappended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more thorough understanding of the present invention, andadvantages thereof, reference is now made to the following descriptionstaken in conjunction with the accompanying drawings, in which:

FIG. 1 shows a cold field electron source embodying the presentinvention.

FIG. 2 is an enlarged, top view of the emitter of the cold field emitterof FIG. 1.

FIG. 3 is a flow chart showing a preferred method of making and usingthe cold field electron source of FIG. 1.

FIG. 4 is a flow chart showing another preferred method of making andusing the cold field electron source of FIG. 1.

FIG. 5 shows an electron instrument incorporating the cold fieldelectron source of FIG. 1

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

FIG. 1 shows a cold field electron source 100 embodying the invention.Cold field emitter 100 includes a filament 102 that supports and heatsan emitter 104 having a tip 106 from which the electrons are emitted.Tip 106 has as small a radius as can be practically constructed,preferably less than 500 nm, more preferably less than 200 nm, and mostpreferably less than, or about, 100 nm. A heating current can besupplied to filament 102 through electrodes 110 that penetrate a base112. Emitter 104 is heated as explained below as part of its preparationfor operation, but is not typically heated during operation.

Emitter 104 preferably comprises a base material of a single crystal ofa metal oriented typically with the <111>, <100>, <110>, or <310>crystaldirection aligned along the emitter axis. Emitter 104 typicallycomprises tungsten, tantalum, rhenium, molybdenum, iridium, othersimilar metals or alloys of these metals. A preferred emitter basematerial is conductive, non-magnetic, has a low work function, and canbe heated to a high temperature without significantly increasing the tipradius. The emitter base material is coated with a coating material.

A preferred coating material does not readily adsorb background gases,such as oxygen, nitrogen, water vapor, and carbon containing gases. Apreferred coating also has a low sputtering coefficient, that is, whenhit by positive ions, relatively few atoms are released from theemitter. A preferred coating also increases the angular confinement ofthe electron beam. These desirable coating characteristics reduce shortand long term current instabilities. A preferred coating may also reducethe work function of the emitter, although reducing the work function isnot as important for a cold field emitter as for a Schottky emitter. Thecoating material can be grown on the emitter by causing a reactionbetween the material composing the emitter and another materialintroduced into the vacuum chamber, such as oxygen, nitrogen, or carbon.The coating can also be deposited onto the emitter. The coating couldinclude, for example, compounds, such as oxide, nitrides and carboncompounds, of the emitter base material or of, for example, zirconium,titanium, hafnium, yttrium, niobium, vanadium, thorium, scandium,beryllium or lanthanum. The coating may include multiple species, suchas a combination of carbon and oxygen on a tungsten emitter.

A preferred combination of emitter base material and coating provides acold field emitter that will operate at a low temperature, at arelatively high pressure compared to prior art cold field emitters, andprovide greater stability. Preferred emitters of the present inventionoperate stably between about 73 K and 700 K, producing an output currentof between 10 nA and 20 μA using extraction voltages of between about900 V and 6000 V at pressures as high as 10⁻⁸ Torr (1.3×10⁻⁸ mbar) orhigher. These operating parameters, and the other operating parametersprovided below are provided as examples, not as limitations on theinvention.

For example, a preferred emitter can operate at a temperature of lessthan 700 K and a pressure greater than 10⁻¹⁰ Torr (1.3×10⁻¹⁰ mbar) for aperiod of 50 hours or greater with less than plus or minus 5%, or morepreferably less than plus or minus 3%, change in beam current, with abeam current of greater than zero nA . Another preferred embodiment canoperate at a temperature of less than 700 K and a pressure greater than5×10⁻¹⁰ Torr (7×10⁻¹⁰ mbar) for a period of 50 hours or greater withless than plus or minus 5%, or more preferably less than plus or minus3%, change in beam current, with a beam current of greater than zero nA.Another preferred emitter can operate at a temperature of less than 700K and a pressure greater than 10^(×9) Torr (1.3×10⁻⁹ mbar) for a periodof 24 hours with less than plus or minus 5%, or more preferably lessthan plus or minus 3%, change in beam current, with a beam current ofgreater than zero nA. Another preferred emitter can operate at atemperature of less than 700 K and a pressure greater than 5×10⁻⁹ Torr(7×10⁻⁹ mbar) for a period of 24 hours with less than plus or minus 5%,or more preferably less than plus or minus 3%, change in beam current,with a beam current of greater than zero nA. Yet another preferredemitter can operate at a temperature of less than 700 K and a pressuregreater than 10⁻⁸ Torr (1.3×10⁻⁸ mbar) for a period of 24 hours withless than plus or minus 5% change in beam current, with a beam currentof greater than zero nA. The emitter is typically operated withoutheating, and is therefore typically near room temperature duringoperation. The emitter can be heated to prevent adsorbates from stickingto the emitter surface should they land there, but the temperature towhich the emitter is heated during operation is preferably sufficientlylow to prevent such thermionic emission that would significantlyincrease the energy spread of the emitted electrons and to noticeablyreduce the image resolution. That is, the temperature during operationis preferably less than about 700 K and more preferably less than about350 K.

In one embodiment, emitter 104 comprises a single crystal tungsten wirehaving its longitudinal axis along a <111>crystal orientation. Tip 106(FIG. 1) has a radius of curvature of preferably about 100 nm. Theemitter 104 is at least partly coated with an oxide layer, the coatingcovering the emitting surface of the tip. FIG. 2, which is an enlargedtop view of emitter 104, shows facets 250 in the emitter wire to exposethree, high work function {211} crystal planes. Facets 250 form as aresult of the oxidation process. The facets result in a high degree ofconfinement of the emission along the axial <111>crystal direction. Thefacet surfaces can be characterized by their electron emission patternand work function. The {211} crystal planes may be further faceted (i.e.built up) by heating in the presence of the electric field, to furtherconfine the emission along the emitter axis.

The oxide layer or other preferred coating has sufficient coverage so asto reduce the adsorption of residual gases on the emitting surface. Theresult is a relatively high brightness, cold field electron source thatprovides stable electron emission in vacuum levels readily achievable inmany existing electron columns.

Applicants have operated an emitter of the present invention for longerthan 500 hours at a pressure of 5×10⁻¹⁰ Torr (7×10⁻¹⁰ mbar) without acontrolled feedback loop and with a relatively stable emission currentof about 2 uA. Under some operating conditions, however, the emissioncurrent may become unstable after an extended period of time. If theemission becomes unstable, thermal processing will restore the emitterto its original emission characteristics. For example, if the emittercoating was originally formed by exposing the emitter base material to acoating component in the form of a gas and then annealing the emitter, a“refresh” step may be performed by heating the emitter to the sametemperature that was used in the original annealing step, and for thesame duration. Depending on the materials of the emitter and thecoating, it may be desirable to expose the emitter to the coatingcomponent again before or during heating in the refresh step. Forexample, if the emitter was originally formed by exposing a tungstenwire to an oxygen atmosphere and then annealing the emitter at about1400 K for about 60 seconds, the refresh step preferably also entailsheating the emitter to about 1400 K for about 60 seconds, with orwithout exposing the emitter to oxygen before or during the refreshannealing step.

The preferred process for producing a particular coating can bedetermined empirically, for example, by varying the coating process andobserving the emission and operating characteristics of the emitterafter each variation. Emission characteristics are observed, forexample, from field emission microscopy (“FEM”) images and from currentversus voltage (“I/V”) curves. Operating characteristics refer toperformance over a longer period of time and are observed by operatingthe emitter for an extended period of time under normal operatingconditions.

For example, to determine a process for producing a suitable oxidecoating on a tungsten base emitter material a tungsten emitter is firstcleaned by heating it briefly to a high temperature of about 2,200 K.After allowing the emitter to return to a temperature of between 300 Kand 1800 K, it is then briefly exposed to oxygen to chemisorb oxygen orother coating components onto the emitter surface, and the emitter isthen annealed for a first period of time, for example, 60 seconds, at afirst temperature, for example, 700 K. The term “anneal” is used hereinto mean heating the emitter to produce the desired coating. A FEM imageis observed and I/V curves are plotted. The process is repeated, thatis, the tungsten needle is cleaned again by briefly heating to a hightemperature, exposed to oxygen, and heated again for the first timeperiod at the first temperature. The emission characteristics areobserved again. If desirable emission characteristics cannot berepeatedly achieved, the heating time and temperature are probablyunsuitable for the materials selected. The emitter preparation processis then varied by varying the time and temperature until desirableemission characteristics are repeatedly achieved. For example, theemitter may be annealed at 800 K or 900 K after exposure to oxygen.After a process is found to repeatedly provide favorable emissioncharacteristics, the operating characteristics of the emitter areobserved by operating the emitter for an extended period of time. If theemitter exhibits suitable operating characteristics, then the coatingprocess can be used to produce additional emitters. The solid angle ofemission can be further confined by building up facets by applyingtemperature and high field simultaneously. This is more commonlyreferred to as a built-up emitter.

FIG. 3 is a flow chart showing a preferred method of making a typicalinventive cold field emitter such as the one shown in FIGS. 1 and 2. Instep 302, a single crystal wire is welded to a filament 102, which hasbeen previously welded to electrodes 110, which extend through base 112.In step 306, the single crystal wire is formed into an emitter byforming a very narrow point, that is, a very small radius tip on the endof the wire. For example, a single crystal tungsten wire, oriented withits axis in the <111>crystal direction can be electrochemically etched,preferably using a direct current etching method, to form a radius ofpreferably less than 200 nm. The tip can be also be formed using analternating current etch method of using a loop method or emitter pullmethod, as taught in A. J. Melmed, “The Art And Science And OtherAspects Of Making Sharp Tips,” Journal of Vacuum Science & Technology B:Microelectronics and Nanometer Structures, Vol. 9, Issue 2, pp. 601-608(1991). In accordance with this method, the relative position of theloop to the emitter is adjusted, or the emitter is pulled from theetching solution as required during the etching to achieve a radius ofpreferably less than 200 nm.

In step 310, the emitter is placed in a charged particle beam system andthe system is evacuated, preferably to a pressure of less than 10⁻⁸ Torr(1.3×10⁻⁸ mbar) and more preferably to a pressure of about 5×10⁻¹⁰ Torr(7×10⁻¹⁰ mbar). In step 312, the emitter is heated briefly, that is, forabout 10 seconds, to about 2,200 K, to remove any contamination oradsorbed gas molecules from the tip. The cleanliness of the tip can beverified by observing an FEM image and by plotting an I/V curve for theemitter.

After the emitter tip is cleaned, a coating is formed at least on theemitter tip by reacting a coating component with the emitter. Forexample, oxygen can be adsorbed onto a tungsten emitter tip and reactedwith the tungsten to form an oxide of tungsten. As shown in branch 314of FIG. 3, the adsorption and reaction can be performed in separatesteps, such as adsorbing the coating component onto the emitter at roomtemperature (step 316) and then heating the emitter to react the coatingcomponent with the emitter (step 318). For a tungsten emitter on whichan oxide coating is being formed, the emitter is preferably heated to atemperature of greater than about 900 K and less than 1,800 K and morepreferably to between about 1,000 K and about 1,200 K. The temperatureshould be sufficiently high to induce the reaction, yet not so high thatthe reaction product dissipates from the emitter. For example, atungsten emitter may be heated to about 1200 K for about 60 seconds toreact adsorbed oxygen to form an oxide.

Alternatively, the coating may be formed in a single step, as shown instep 322, by maintaining the emitter at a sufficiently elevatedtemperature to induce the reaction while introducing the coatingcomponent. In steps 316 and 322, oxygen for example may be leaked in tothe vacuum chamber to a partial pressure of about 10⁻⁶ Torr (1.3×10⁻⁶mbar) for about 3 minutes. The oxygen is then removed from the vacuumchamber by a vacuum pump.

In step 316, the adsorption of oxygen at room temperature forms achemisorbed layer. Heating in step 318 transforms the chemisorbed layerof oxygen into an oxidized layer. While two processes are describedabove for forming a coating, other methods are known for formingcoatings, and the invention is not limited to the methods describedabove. After the coating is formed, by whatever process, the emitter isnow ready to be used.

In the steps 318 and 322, the emitter is typically heated by running acurrent through the filament. Voltage is not typically applied to theemitter during the steps 318 and 322. In step 324, an extraction voltageof, for example, about 2000 V is applied between the filament and anextraction electrode (not shown) to begin extracting electrons from theemitter for normal operation. The extraction voltage is preferablybetween 1000 V and 3000 V and more preferably between 1000 V and 2000 V.Skilled persons will understand that the operating voltage will varywith the tip radius and other factors, such as tip material, crystalorientations, coating properties, emitter-counter electrode regiongeometry, and the presence of tip contaminants.

An emitter of the present invention can be used in any electron beamsystem, including microscopes and spectrometers, such as scanningelectron microscopes, transmission electron microscopes, scanningtransmission electron microscope, Auger electron spectrometers, electronenergy loss spectrometer, energy dispersive spectrometers, etc. Overtime, the emitter tip may become contaminated by residual gases. Inoptional step 330, the emitter is heated to clean the tip and restorethe original emission characteristics. For example, the emitter may beheated to same temperature as was used in the initial annealing step,step 318 or 322, for about 60 seconds or for the duration of the initialannealing step. Depending on the construction of the emitter, theemitter may be exposed again to a coating component, such as oxygen,before step 330.

FIG. 4 is a flow chart showing another embodiment of the invention whichproduces an emitter in which emission is confined to a relatively smallarea and provides a surface layer that reduces the adsorption ofresidual gases. As shown in FIG. 4, a preferred method for forming sucha structure includes providing a field emitter with a <111>orientationin step 400 and then cleaning the emitter by heating to 1800 K for 10 to15 sec in step 402. In step 404, the emitter is exposed to a pure oxygenpressure of about 1×10⁻⁶ torr (1.7×10⁻⁶ mbar) for several minutes atroom temperature. In step 406, the emitter is heated to 1600 K for 30 to60 sec.

Performing steps 400 to 406 will produce an emitted having emissionconfined to a small solid angle centered on the axis of the emitter.Adsorbing oxygen on a tungsten (111) planar surface and heating it to anelevated temperature causes an array of 3-sided pyramids to form with{112} planes making up the sides of the pyramid. These pyramids growbecause of the lower surface free energy of the {112} planes; uponheating to allow for surface mobility, the latter planes grow at theexpense of the (111) plane. The {112} planes form a thin WO_(x) oxidelayer (x is unknown). The oxide layer will reduce the adsorption ofresidual gases on the emitter surface and improve emission stability atroom temperature. Heating to a sufficiently higher temperatures,however, removes the WO_(x) layer and restores the planar (111) surface.

The emission can be further confined to a smaller solid angle by heatingin optional step 408 the emitter in the presence of a field sufficientto obtain a few microamps of total current at a temperature of 1200 to1600 K. The process by which this occurs is a faceting of the three{112} planes surrounding the axial (111) plane. The emitter can then beoperated in step 410 stably at room temperature or at some elevatedtemperature provided the vacuum level is sufficiently low. If theemission becomes unstable, stable emission can be obtained in step 412by re-heating the emitter at 1200 to 1400 K for several seconds. FIG. 4provides an example with specific times and temperatures, but theinvention is not limited to the processing parameters described inconnection with FIG. 4. A person of skill in the art can use the exampleof FIG. 4 to create other processes which use different times andtemperatures, but are within the scope of the present invention.

FIG. 5 shows a block diagram of an electron beam system 502, such as ascanning electron microscope, that incorporates a cold field electronsource 100 of the present invention. Cold field electron source 504 ismaintained within a vacuum chamber 506 that also includes an electroncolumn 508, a secondary particle detector 510, and a specimen 512 on astage 514. Controller 520 controls the components in the system 502 todirect an electron beam toward specimen 512 and to display an image ofthe work piece on a display 522 using secondary particles detected bydetector 510. Electron beam system 502 is provided by way of example,and cold emitters in accordance with the invention are not limited intheir application to the configuration shown.

Embodiments of the present invention can increase the resolution of theelectron microscope by providing a source that is bright and has a lowenergy spread. Embodiments can operate reliably at a pressure greaterthan that in which a prior art cold field emitter can operate reliably.Embodiments can also provide improved stability when operated atpressures similar to those used for conventional cold field emitters.Embodiments can also provide various analytical capabilities such aselectron energy loss spectroscopy. Some embodiments of cold fieldemitters of the present invention can operate at vacuum levels that aretypical for Schottky emitters. Emitters of the present invention can beused in almost any application in which electron emitters are requiredand are used not only in electron beam systems, but in dual beam orother multiple beam systems.

While the specification describes emitters that can operate atconditions that were unsuitable for prior art cold field emitters,emitters of the present invention can obviously also be operated underthe more ideal conditions that were required by prior art cold fieldemitters. That is, embodiments of the invention have certaincapabilities, but the invention still covers those embodiments even whenthose capabilities are not being utilized. Embodiments of the inventionprovide an angular electron emission that is sufficient for typicallyelectron beam applications.

Although the present invention and its advantages have been described indetail, it should be understood that various changes, substitutions andalterations can be made herein without departing from the spirit andscope of the invention as defined by the appended claims. Moreover, thescope of the present application is not intended to be limited to theparticular embodiments of the process, machine, manufacture, compositionof matter, means, methods and steps described in the specification. Asone of ordinary skill in the art will readily appreciate from thedisclosure of the present invention, processes, machines, manufacture,compositions of matter, means, methods, or steps, presently existing orlater to be developed that perform substantially the same function orachieve substantially the same result as the corresponding embodimentsdescribed herein may be utilized according to the present invention.Accordingly, the appended claims are intended to include within theirscope such processes, machines, manufacture, compositions of matter,means, methods, or steps.

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 10. A method ofemitting electrons using a cold filed emitter, comprising: providing abase; providing multiple electrodes extending through the base;providing a filament connected to two of the electrodes; providing anemitter attached to the filament, the emitter having at one end a singlecrystal emitter tip of an emitter tip material; providing a layer of acoating material on the emitter tip, the layer of coating materialincluding a chemical combination of the material of which the emittertip is constructed and another material; and applying an extractionvoltage near the emitter tip to extract electrons while maintaining theemitter tip at a temperature such that no significant thermionicemission occurs.
 11. The method of claim 10 in which providing a layerof a coating material on the emitter tip includes exposing a tungstenemitter tip to oxygen or nitrogen and then heating the tip sufficientlyto react the oxygen or nitrogen with the tungsten.
 12. The method ofclaim 10 in which providing an emitter includes providing a singlecrystal emitter tip having a longitudinal axis oriented in the<111>crystal direction.
 13. The method of claim 12 in which providing alayer of a coating material includes heating the emitter tip atemperature sufficient to form an array of pyramids defined by {112}planes.
 14. The method of claim 10 in which providing a layer of acoating material on the emitter tip includes providing an oxide ornitride of the material composing the emitter.
 15. The method of claim10 in which providing a layer on the emitter tip includes providing alayer of a coating material comprising multiple species.
 16. The methodof claim 15 in which providing a layer a coating material on the emittertip includes providing a layer a coating material comprising carbon andoxygen.
 17. The method of claim 10 in which the providing a layer acoating material on the emitter tip includes providing a layer a coatingmaterial having a coverage of less than a monolayer.
 18. The method ofclaim 10 further comprising heating the emitter while extractingelectrons to further confined the emission to the tip of the emitterbefore operating the emitter.
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 25. A methodof producing a cold field emitter, comprising: providing a wire having aportion tapering toward a tip; exposing the wire to gas at a lowpressure to adsorb gas molecules onto the wire; and heating the wireafter exposing the wire to a gas at a low pressure to a temperaturesufficient to react the adsorbed gas molecules with the wire to form acoating improves the stability of the emission at low temperatures. 26.The method of claim 25 in which heating the wire after exposing the wireto a gas at a low pressure to a temperature sufficient to react theadsorbed gas molecules with the wire includes forming low emissionplanes so that emission from the wire are concentrated at the tip of thewire.
 27. The method of claim 25 further comprising heating the emitterto above operating temperature while extracting electrons to furtherconfined the emission to the tip of the emitter after heating the wireafter exposing the wire to a gas at a low pressure and before operatingthe emitter.
 28. The method of claim 25 in which applying an electricalpotential between the wire and an extraction electrode to cause the wireto emit electrons at a temperature below the temperature at whichsignificant thermionic emission occurs includes applying an electricalpotential between the wire and an extraction electrode to cause the wireto emit electrons at a temperature below 700 K.
 29. The method of claim25 further comprising heating the wire to at least 1500 K beforeexposing the wire to gas at low pressure and in which: providing a wireincludes providing a tungsten wire; exposing the wire to gas at a lowpressure includes exposing the wire to oxygen at a partial pressure ofless than 10⁻⁴ Torr (1.3×10⁻⁴ mbar); and heating the wire includesheating the wire to greater than 1000 K.
 30. The method of claim 29 inwhich providing a wire includes providing a tungsten wire includesproviding single crystal tungsten wire having a tip radius of less than400 nm and a longitudinal axis oriented in the <111>crystal direction.31. The method of claim 30 in which providing a wire includes providinga single crystal wire tapering toward a tip having a radius of less than200 nm.
 32. The method of claim 31 in which providing a wire includesproviding a tungsten wire having a longitudinal axis oriented in the<111>direction.
 33. The method of claim 25 further comprising applying avoltage to the emitter to operate the emitter at a temperature of lessthan 700 K in a pressure of greater than 5×10⁻⁹ Torr (7×10⁻⁹ mbar) forlonger than 50 hours with less than plus or minus 5% change in beamcurrent without an external feedback control loop, with a beam currentof greater than zero nA.
 34. A method of operating a cold field electronemitter, comprising: providing an emitter in a vacuum chamber, theemitter tip including a coating over a base material; evacuating thevacuum chamber to a pressure of equal to or greater than about 10⁻⁹ Torr(1.3×10⁻⁹ mbar); and providing an electric field at the tip of theemitter to extract electrons, the emitter temperature being below atemperature at which significant thermionic emission occurs, to extractelectrons continuously for a period of greater 24 hours with a beamcurrent variation of less than 5% without an external feedback controlloop, with a beam current of greater than zero nA.
 35. The method ofclaim 34 in which providing an electric field at the tip of the emitterto extract electrons, the emitter temperature being below a temperaturewhich significant thermionic emission occurs includes providing anelectric field at the tip of the emitter to extract electrons, theemitter temperature being below 700 K.
 36. The method of claim 25 inwhich heating the wire to react the adsorbed gas molecules includesheating the wire in the absence of an electric field applied to theemitter tip.
 37. The method of claim 25 in which heating the wire toreact the adsorbed gas molecules includes heating the wire for between30 seconds and 60 seconds